Polymer Film Dewetting by Water/Surfactant/Good-Solvent Mixtures: A Mechanistic Insight and Its Implications for the Conservation of Cultural Heritage

New sustainable polymers and oligomers for Cultural Heritage conservation

The development of "green" chemistry materials with enhanced properties is a central topic in numerous applicative fields, including the design of polymeric systems for the conservation of works of art. Traditional approaches in art restoration comprise polymer thickeners and viscous dispersions to partially control solvents in the removal of soil or aged varnishes/coatings from artifacts. Alternatively, polymeric gel networks can be specifically designed to grant full control of the cleaning action, yielding safe, time- and cost-effective restorations. The selection of polymers and oligomers in gel design is crucial to tune solvent upload, retention, and controlled release over the sensitive artistic surfaces. Starting from an overview of traditional polymer formulations and state-of-the-art gel systems for cleaning works of art, we provide here the design of a new class of gels, focusing on the selection of oligomers to achieve gels with tailored hydrophilicity/hydrophobicity. We evaluated the oligomers Hydrophilic-Lipophilic Balance (HLB) by developing, for the first time, a novel methodology combining SEC and DOSY NMR analysis, which was tested on a library of "green" oligoesters synthesized by polycondensation and poorly explored in the literature. Oligomers with moderate polydispersity were chosen to validate the new protocol as a robust tool for designing polymeric gels even on industrial scale. The methodology is more time-effective than traditional methods, and gives additional insights on the oligomers physico-chemical nature, evaluating their compatibility with different solvents. Then, we used the selected oligoesters with castor oil to obtain a new class of organogels able to upload solvents with varying polarity, which effectively removed different types of unwanted layers typically found in painting restoration. These results validate the oligomers screening approach and the new class of gels as promising chemical processes/materials in art preservation. The methodology can potentially allow evaluation of HLB also for small molecules (e.g., surfactants), opening for the formulation of polymers solutions/gels beyond Cultural Heritage conservation, as in pharmaceutics, cosmetics, food industry, tissue engineering, agriculture, and others.

Enhanced Removal of Photoresist Films through Swelling and Dewetting Using Pluronic Surfactants

Organic photoresist coatings, primarily composed of resins, are commonly used in the electronics industry to protect inorganic underlayers. Conventional photoresist strippers, such as amine-type agents, have shown high removal performance but led to environmental impact and substrate corrosiveness. Therefore, this trade-off must be addressed. In this study, we characterized the removal mechanism of a photoresist film using a nonionic triblock Pluronic surfactant [poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide)] in a ternary mixture of ethylene carbonate (EC), propylene carbonate (PC), and water. In particular, the removal dynamics determined by using a quartz crystal microbalance with dissipation monitoring was compared with those determined by performing confocal laser scanning microscopy and visual observation to analyze the morphology, adsorption mass, and viscoelasticity of the photoresist film. In the absence of the Pluronic surfactant, the photoresist film in the ternary solvent exhibited a three-step process: (i) film swelling caused by the penetration of a good solvent (EC and PC), (ii) formation of photoresist particles through dewetting, and (iii) particle aggregation on the substrate. This result was correlated to the Hansen solubility parameters. The addition of the Pluronic surfactant not only prevented photoresist aggregation in the third step but also promoted desorption from the substrate. This effect was dependent on the concentration of the Pluronic surfactant, which influenced diffusion to the interface between the photoresist and the bulk solution. Finally, we proposed a novel photoresist stripping mechanism based on the synergy between dewetting driven by an EC/PC-to-water mixture and adsorption by the Pluronic surfactant.

From Nanoparticles to Gels: A Breakthrough in Art Conservation Science

Cultural heritage is a crucial resource to increase our society's resilience. However, degradation processes, enhanced by environmental and anthropic risks, inevitably affect works of art, hindering their accessibility and socioeconomic value. In response, interfacial and colloidal chemistry has proposed valuable solutions over the past decades, overcoming the limitations of traditional restoration materials and granting cost- and time-effective remedial conservation of the endangered artifacts. Ranging from inorganic nanoparticles to hybrid composites and soft condensed matter (gels, microemulsions), a wide palette of colloidal systems has been made available to conservators worldwide, targeting the consolidation, cleaning, and protection of works of art. The effectiveness and versatility of the proposed solutions allow the safe and effective treatment of masterpieces belonging to different cultural and artistic productions, spanning from classic ages to the Renaissance and modern/contemporary art. Despite these advancements, the formulation of materials for the preservation of cultural heritage is still an open, exciting field, where recent requirements include coping with the imperatives of the Green Deal to foster the production of sustainable, low-toxicity, and environmentally friendly systems. This review gives a critical overview starting from pioneering works up to the latest advancements in colloidal systems for art conservation, a challenging topic where effective solutions can be transversal to multiple sectors even beyond cultural heritage preservation, from the pharmaceutical and food industry, to cosmetics, tissue engineering, and detergency.

Polyelectrolyte Coatings-A Viable Approach for Cultural Heritage Protection

The continuous degradation of cultural heritage artifacts (due to different factors, including the rising air pollution, climate change or excessive biological activity, among others) requires the continuous development of protection strategies, technologies and materials. In this regard, polyelectrolytes have offered effective ways to fight against degradation but also to conserve the cultural heritage objects. In this review, we highlight the key developments in the creation and use of polyelectrolytes for the preservation, consolidation and cleaning of the cultural heritage artifacts (with particular focus on stone, metal and artifacts of organic nature, such as paper, leather, wood or textile). The state of the art in this area is presented, as well as future development perspectives.

Preparation of Surface-Supported Polylactide Spherical-Cap Particles

Biodegradable polymer particles are of considerable importance due to their multiple applications in medical diagnostics and therapy. Spherical-cap particles have been prepared in a very general and simple method by melting a thin polymer film supported on a solid substrate that is in contact with a hydrophilic solvent. The melted polymer forms droplets which transform into solid particles attached to the surface after cooling down the sample. This approach has been demonstrated for polylactide adlayers on glass, which, when melted in glycerol, produce an array of polymer particles supported on the surface. The size of the particles depends on the experimental conditions and ranges from tens of nanometers to several micrometers. The particles can be employed to incorporate guest species, for example, drug molecules or inorganic nanoparticles. This has been confirmed herein through entrapment of an anticancer drug (doxorubicin) and radiogold (Au-198) nanoparticles. The resulting structures have been examined using a number of complementary physicochemical techniques including scanning and transmission electron microscopy, atomic force and optical microscopy as well as Raman and fluorescence spectroscopy.

Insight into a Bentonite-Based Hydrogel for the Conservation of Sandstone-Based Cultural Heritage: In Situ Formation, Reinforcement Mechanism, and High-Durability Evaluation

Conservation of sandstone-based cultural heritage has attracted a great deal of interest. We propose herein a novel protecting strategy, via in situ fabrication of bentonite-based hydrogels (B-H) inside sandstones, where the bentonite-based hydrogels serve as the underlying cement. To create bentonite-based hydrogels with controllable structure, possessing good mechanical and anti-swelling properties, we have optimized forming time, appearance, and viscosity. The hydrogel precursor penetrated into the pores of the sandstone; the hydrogel would then form within 3-5 h. As found by employing a fluorescent tracer, the precursor remained controllably in place without any apparent change in the sandstone morphology. The bentonite-based hydrogels that formed inside the sandstones presented strong hydrogen bonding, coordination, and ionic bonding, as well as strong mechanical interlocking to the sandstone matrix. As a result, the sandstones possessed enhanced mechanical compressive strength and excellent resistance to acid, salt, and freeze-thaw cycles. Our approach provides for a non-destructive, eco-friendly, easy-to-use, and long-term strategy for cultural preservation, one with excellent protection effects.

Self-Shaping Microemulsion Gels for Cultural Relic Cleaning

Cleaning is a foundational and essential operation of protection and restoration of cultural relics, which is also the key step of follow-up works. To overcome the problems of uncontrollable diffusion of cleaning solvents and poor coverage of the cleaning solvent carriers on rough surfaces, here, we propose a strategy of using a self-shaping microemulsion gel that is prepared via emulsifying oleophilic solvents into the specific shear-thinning hydrogel structures. The gel can adaptively cover rough surfaces during the cleaning process coupled with avoidance of unnecessary diffusion of the cleaning solvents, and the mechanical reinforcement of in situ polymerized double-network gels enables its easy peeling off from the surfaces without leaving determinable residues. As a representative demonstration, Paraloid B72, a widely used material for the repair and reinforcement of cultural relics, is employed as a model discolored coating, which can be effectively removed from the rough surface of simulated cultural relics after treatment with the resulting gels. Convincingly, the strategy of constructing agarose/polyacrylamide hybrid double-network gels with shear-thinning and self-shaping performances for the cleaning of cultural relics not only improves the convenience and accuracy of operation but also exhibits an efficient cleaning effect, which will greatly expand the application of microemulsion gels in the cleaning of rough surfaces of cultural relics.

Formulation of Environmentally Safe Graffiti Remover Containing Esterified Plant Oils and Sugar Surfactant

The removal of graffiti or over-painting requires special attention in order to not induce the surface destruction but to also address all of the important eco-compatibility concerns. Because of the necessity to avoid the use of volatile and toxic petroleum-based solvents that are common in cleaning formulations, much attention has recently been paid to the design of a variety of sustainable formulations that are based on biodegradable raw materials. In the present contribution we propose a new approach to graffiti cleaning formulations that are composed of newly synthesized green solvents such as esterified plant oils, i.e., rapeseed oil (RO), sunflower oil (SO), or used cooking oil (UCO), ethyl lactate (EL), and alkylpolyglucosides (APGs) as surfactants. Oil PEG-8 ester solvents were synthesized through the direct esterification/transesterification of these oils using monobutyltin(IV) tris(2-ethylhexanoate) and titanium(IV) butoxide catalysts under mild process conditions. The most efficient formulations, determined by optimization through the response surface methodology (RSM) was more effective in comparison to the reference solvents such as the so-called Nitro solvent (denoting a mixture of toluene and acetone) and petroleum ether. Additionally, the optimal product was found to be effective in removing graffiti from glass, metal, or sandstone surfaces under open-field conditions in the city of Wrocław. The performed studies could be an invaluable tool for developing future green formulations for graffiti removal.

Nanostructured fluids for polymeric coatings removal: Surfactants affect the polymer glass transition temperature

HYPOTHESIS

Nanostructured fluids (NSFs) based on water, organic solvents and surfactants are a valid alternative to the use of neat unconfined organic solvents for polymer coatings removal in art conservation. The physico-chemical processes underpinning their cleaning effectiveness in terms of swelling/dewetting of polymer films were identified as key in this context. The role of surfactants on polymers' dewetting was considered to be mainly restricted to the lowering of interfacial tensions. However, recent experiments evidenced that surfactants have an important role in swelling polymer films.

EXPERIMENTS

Five different amphiphiles were selected, namely: sodium dodecylsulfate, dimethyldodecyl amine oxide, hexaoxyethylene decyl ether (C_9-11E₆), pentadecaoxyethylene dodecyl ether (C₁₂E₁₅), and methyoxypentadecaoxyethylene dodecanoate (C₁₁COE₁₅CH₃). They were combined with a carefully selected organic solvents' mixture (1-butanol/butanone/dimethyl carbonate) to formulate new NSFs, differing for the surfactant only, and used to perform cleaning tests on surfaces coated with Paraloid B72® and Primal AC33®. Here for the first time, polymer swelling induced by surfactants was quantified and correlated with the glass transition temperature of the two polymers by differential scanning calorimetry, before and after the exposure to the fluids. Confocal laser scanning microscopy and small-angle X-ray scattering provided additional insights on the interaction mechanism.

FINDINGS

Nonionics were proven more efficient than zwitterionic/ionic amphiphiles in the polymer swelling, and, overall, methyoxy pentadecaoxyethylene dodecanoate resulted the most effective among the selected surfactants. A direct relation between the effect of surfactants on the polymers' glass transition temperature and cleaning capacity was established. This finding, fundamental to understand the interaction mechanism between NSFs and polymer coatings or paint layers, is key to achieve a selective, effective and complete removal of polymer coatings, as recently shown in the removal of vandalism and over-paintings from street art.

How Science Can Contribute to the Remedial Conservation of Cultural Heritage

Colloid science is contributing solutions to counteract the degradation of artifacts, favoring their transfer to future generations. Advanced materials such as nanoparticles, coatings, gels and microemulsions have been assessed in conservation, spanning from archeological sites to modern and contemporary art. We give an overview of the fundamental milestones and latest innovations in conservation science, targeting solutions and tools for remedial conservation based on green nanomaterials and hybrid systems. Future perspectives and outstanding challenges in this exciting field are then outlined.

Advanced Materials in Cultural Heritage Conservation

Cultural Heritage is a crucial socioeconomic resource; yet, recurring degradation processes endanger its preservation. Serendipitous approaches in restoration practice need to be replaced by systematically addressing conservation issues through the development of advanced materials for the preservation of the artifacts. In the last few decades, materials and colloid science have provided valid solutions to counteract degradation, and we report here the main highlights in the formulation and application of materials and methodologies for the cleaning, protection and consolidation of works of art. Several types of artifacts are addressed, from murals to canvas paintings, metal objects, and paper artworks, comprising both classic and modern/contemporary art. Systems, such as nanoparticles, gels, nanostructured cleaning fluids, composites, and other functional materials, are reviewed. Future perspectives are also commented, outlining open issues and trends in this challenging and exciting field.

Ultrasound-Stimulated PVA Microbubbles for Adhesive Removal from Cellulose-Based Materials: A Groundbreaking Low-Impact Methodology

In this work, we shed new light on ultrasound contrast agents applied to the field of cultural heritage as an invaluable fine-tune cleaning tool for paper artworks. In this context, one of the primary and challenging issues is the removal of modern adhesives from paper artifacts. Modern adhesives are synthetic polymers whose presence enhances paper degradation and worsens its optical features. A thorough analytical and high-spatial-resolution combined study was successfully performed to test the capability of poly(vinyl alcohol)-based microbubbles stimulated by a proper noninvasive 1 MHz ultrasound field exposure in removing these adhesives from paper surfaces, in the absence of volatile invasive and toxic chemicals and without damaging paper and/or leaving residues. We demonstrate that poly(vinyl alcohol)-shelled microbubbles are suitable for interacting with paper surfaces, targeting and boosting in a few minutes the nondamaging removal of adhesive particles from paper samples thanks to their peculiar shell composition together with their ultrasound dynamics.

Selective removal of over-paintings from "Street Art" using an environmentally friendly nanostructured fluid loaded in highly retentive hydrogels

HYPOTHESIS

The removal of over-paintings or graffiti is a priority for conservators and restorers. This operation is complex, especially when over-paintings lay on painted surfaces that must be preserved, as in the case of vandalism on street art, where the layers are usually chemically similar. Traditional methodologies often do not provide satisfactory results and pose health and eco-compatibility concerns. An alternative methodological approach based on an environmentally friendly nanostructured fluid loaded in a retentive hydrogel is here proposed.

EXPERIMENTS

Six paints (based on vinyl, acrylic and alkyd polymers) were selected and studied by means of attenuated total reflection - Fourier transform infrared spectroscopy. The phase behavior of four alkyl carbonates (green, low-toxicity organic solvents) and a biodegradable nonionic surfactant in water was investigated with Small angle X-ray scattering (SAXS) in order to formulate a novel nanostructured cleaning system. The developed system, which also includes 2-butanol and an alkyl glycoside hydrotrope, was loaded in highly retentive hydrogels and tested in the selective removal of over-paintings from laboratory mockups and from real pieces of street art.

FINDINGS

The selective and controlled removal of modern paints from substrates with similar chemical composition has been achieved using a specifically tailored NSF embedded in a retentive hydrogel. The proposed methodology and cleaning system provided excellent cleaning results, representing a new tool for the conservation of contemporary and, in particular, street art.

The Adverse Effects of TiO2 Photocatalycity on Paraloid B72 Hybrid Stone Relics Protective Coating Aging Behaviors under UV Irradiation

The incorporation of photocatalytic nanomaterials into polymer coatings is used to protect stone relics from weathering. However, the photocatalytic nanomaterials might generate excess free radicals to degrade the polymer matrix. In this work, a certain amount of TiO₂ nanoparticles were dispersed into Paraloid B72 and applied onto sandstone relics to explore the adverse effects of TiO₂ nanoparticles on Paraloid B72 under ultraviolet (UV) irradiation. To fulfill this goal, the effects of TiO₂ on pore formation and the structure of Paraloid B72 was studied by scanning electron microscopy (SEM). Moreover, the surface chemical composition, pore structure, surface roughness and surface wettability were explored via Fourier transform infrared (FTIR) spectroscopy, SEM, optical profilometer and water contact angle measurement under UV irradiation. Results showed that the incorporation of TiO₂ nanoparticles prohibited the generation of pores in Paraloid B72 and there were no pores formed when the content of TiO₂ exceeded 0.8 wt%. The water contact angle of origin Paraloid B72 and TiO₂/Paraloid B72 decreased with the prolonging UV irradiation. Moreover, TiO₂ nanoparticles were extracted from the matrix and the pores cannot be detected with the prolonging UV irradiation time under a higher content of TiO₂. These research findings might promote the understanding of using photocatalytic nanomaterials in developing stone relics' protective coating.

Nonionic Surfactants for the Cleaning of Works of Art: Insights on Acrylic Polymer Films Dewetting and Artificial Soil Removal

The use of nanostructured fluids (NSFs), that is, micellar solutions and microemulsions, in art conservation is often associated with cleaning purposes as the removal of polymeric coatings and/or soil from artistic surfaces. In both cases, the use of NSFs grants significant improvements over the use of traditional cleaning techniques that employ neat unconfined organic solvents, water, or aqueous solutions. The study of the nature and properties of surfactants present in NSF formulations is important to boost the effectiveness of these systems in applicative contexts and in the search of innovative and highly performing amphiphiles. This work reports on the methoxy-pentadeca(oxyethylene) dodecanoate (MPD) surfactant in two different NSFs, whose utilization in conservation of cultural heritage is new. Its effectiveness is compared to the conventional nonionic amphiphiles used in conservation practice, as pentadeca(oxyethylene) dodecyl ether, for the cleaning of poly(ethyl methacrylate/methyl acrylate) 70:30, p(EMA/MA), and artificially soiled surfaces. The mechanism, through which NSFs interact with polymeric coatings or soiled surfaces, was investigated by confocal laser scanning microscopy, fluorescence correlation spectroscopy, photographic observation, contact angle, surface tension measurements, and small-angle X-ray scattering. The results highlighted the superior MPD's performance, both in inducing polymer removal and in detaching the soil from coated surfaces. At the microscale, the cleaning involves dewetting-like processes, where the polymer or the soil oily phase is detached from the surface and coalesce into separated droplets. This can be accounted by considering the different surface tensions and the different adsorption mechanisms of MPD with respect to ordinary nonionic surfactants (likely due to the methyl capping of the polar head chain and to the presence of the ester group between the hydrophilic and hydrophobic parts of the MPD surfactant molecule), showing how a tiny change in the surfactant architecture can lead to important differences in the cleaning capacity. Overall, this paper provides a detailed description of the mechanism and the kinetics involved in the NSFs cleaning process, opening new perspectives on simple formulations that are able to target at a specific substance to be removed. This is of utmost importance in the conservation of irreplaceable works of art.

Twin-chain polymer hydrogels based on poly(vinyl alcohol) as new advanced tool for the cleaning of modern and contemporary art

Conservation of our cultural heritage is fundamental for conveying to future generations our culture, traditions, and ways of thinking and behaving. Cleaning art, in particular modern/contemporary paintings, with traditional tools could be risky and impractical, particularly on large collections of important works to be transferred to future generations. We report on advanced cleaning systems, based on twin-chain polymer networks made of poly(vinyl alcohol) (PVA) chains, semiinterpenetrated (semi-IPN) with PVA of lower molecular weight (L-PVA). Interpenetrating L-PVA causes a change from gels with oriented channels to sponge-like semi-IPNs with disordered interconnected pores, conferring different gel (and solvent) dynamics. These features grant residue-free, time efficient cleaning capacity and effective dirt capture, defeating risks for the artifact, making possible a safer treatment of important collections, unconceivable with conventional methods. We report as an example the conservation of Jackson Pollock's masterpieces, cleaned in a controlled way, safety and selectivity with unprecedented performance.

Guiding the development of sustainable nano-enabled products for the conservation of works of art: proposal for a framework implementing the Safe by Design concept

Nanotechnology provides innovative and promising solutions for the conservation of cultural heritage, but the development and application of new nano-enabled products pose concerns regarding their human health and environmental risks. To address these issues, we propose a sustainability framework implementing the Safe by Design concept to support product developers in the early steps of product development, with the aim to provide safer nano-formulations for conservation, while retaining their functionality. In addition, this framework can support the assessment of sustainability of new products and their comparison to their conventional chemical counterparts if any. The goal is to promote the selection and use of safer and more sustainable nano-based products in different conservation contexts. The application of the proposed framework is illustrated through a hypothetical case which provides a realistic example of the methodological steps to be followed, tailored and iterated along the decision-making process.

Hydrogels formed by anammox extracellular polymeric substances: structural and mechanical insights

The recovery of biopolymers from the waste sludge produced in wastewater treatments and their application in other industrial sectors, would substantially increase the environmental and economical sustainability of the process, promoting the development of a circular economy. In this study, extracellular polymeric substances (EPS) extracted from anammox granular waste sludge, were investigated and characterized. Rheological and differential scanning calorimetry measurements on EPS aqueous dispersions indicate the formation of an extended 3-D network above a threshold concentration, with a clear dependence of the mechanical and water retention properties on EPS content. The structural characterization, performed with transmission electron microscopy and small angle X-ray scattering, reveals the presence of functional amyloids as putative structural units, observed for the first time in an EPS-based hydrogel. As a proof of concept of the applicative potential, we explored the water and grease resistance provided to paper by an EPS coating. These results shed light on the structural details of EPS-based hydrogels, and pave the way for the possible use of EPS-based materials as a cheap, eco-friendly alternative to commonly adopted paper coatings, in line with a circular economy pattern for wastewater treatment.

Surfactants Mediate the Dewetting of Acrylic Polymer Films Commonly Applied to Works of Art

The removal of hydrophobic polymer coatings from artistic surfaces is a ubiquitous challenge in art restoration. Over the years, nanostructured fluids (NSFs), aqueous surfactant solutions containing a good solvent for the polymer, have been successfully applied in polymer removal interventions; however, the precise role of the surfactant in promoting polymer film dewetting is not fully understood. This contribution addresses the interaction of a NSF of water/propylene carbonate containing a nonionic surfactant with an acrylic polymer film commonly used in art conservation. Combining confocal microscopy and fluorescence correlation spectroscopy, we monitored the penetration of the fluid into the polymer film, defining its compositional changes and following the polymer swelling. The ensemble of results highlights that the surfactant role is twofold: (i) at the polymer-support interface, it promotes the detachment of the polymer film from the underlying support; (ii) inside the polymer film, it accelerates polymer swelling by increasing the chains' mobility.

The carbonation kinetics of calcium hydroxide nanoparticles: A Boundary Nucleation and Growth description

HYPOTHESIS

The reaction of Ca(OH)₂ with CO₂ to form CaCO₃ (carbonation process) is of high interest for construction materials, environmental applications and art preservation. Here, the "Boundary Nucleation and Growth" model (BNGM) was adopted for the first time to consider the effect of the surface area of Ca(OH)₂ nanoparticles on the carbonation kinetics.

EXPERIMENTS

The carbonation of commercial and laboratory-prepared particles' dispersions was monitored by Fourier Transform Infrared Spectroscopy, and the BNGM was used to analyze the data. The contributions of nucleation and growth of CaCO₃ were evaluated separately.

FINDINGS

During carbonation the boundary regions of the Ca(OH)₂ particles are densely populated with CaCO₃ nuclei, and transform early with subsequent thickening of slab-like regions centered on the original boundaries. A BNGM limiting case equation was thus used to fit the kinetics, where the transformation rate decreases exponentially with time. The carbonation rate constants, activation energies, and linear growth rate were calculated. Particles with larger size and lower surface area show a decrease of the rate at which the non-nucleated grains between the boundaries transform, and an increase of the ending time of Ca(OH)₂ transformation. The effect of temperature on the carbonation kinetics and on the CaCO₃ polymorphs formation was evaluated.

Hybrid nano-composites for the consolidation of earthen masonry

HYPOTHESIS

Earth is one of the oldest silicate-based materials in stone heritage, still largely used in architecture worldwide. Earthen materials are highly susceptible to wind and water erosion, leading to loss of cohesion and crumbling. Conventional consolidants (alkoxysilanes, synthetic or natural polymers) lack physico-chemical compatibility or effectiveness, and can promote degradation. We propose for the first time nano-composites for the surface consolidation of adobe, i.e. unbaked earth bricks often containing organic fibers and lime.

EXPERIMENTS

We investigated, mimicking the setting of portland cement, the formation of calcium silicate hydrate (CSH) within adobe porosities, owing to the pozzolanic reaction between nanoparticles of silica and calcium hydroxide, to consolidate a powdery substrate. Different formulations were characterized by Fourier Transform Infrared spectroscopy (FTIR), X-ray diffraction (XRD), scanning and transmission electron microscopy (SEM, TEM), dynamic light scattering (DLS), and turbidimetry (UV-Vis spectroscopy).

FINDINGS

A ternary composite made of SiO₂ nanoparticles, Ca(OH)₂ nanoparticles, and hydroxypropyl cellulose, dispersed in a (4:1) ethanol:water blend, was formulated. Each component is compatible with adobe, and plays a role in its consolidation. The treatment of adobe samples with the composite leads to the in situ formation of CSH, providing resistance to peeling, abrasion, and wet-dry cycles, with no aesthetic alteration. This opens new perpectives in the preservation of one of the most widely used construction materials.

A Triton X-100-Based Microemulsion for the Removal of Hydrophobic Materials from Works of Art: SAXS Characterization and Application

The removal of hydrophobic materials from a porous support, such as wax stains on wall paintings, is particularly challenging. In this context, traditional methods display several drawbacks. The limitations of these methods can be overcome by amphiphile-based aqueous nanostructured fluids, such as micellar solutions and microemulsions. In this study, a microemulsion for the removal of wax spots from artistic surfaces was formulated. The nanostructured fluid includes a non-ionic surfactant, i.e., Triton X-100, and two apolar solvents, namely p-xylene and n-nonane. The solvents were selected on the basis of solubility tests of three waxes in several organic solvents. The nanostructured fluid was characterized by means of small-angle X-rays scattering (SAXS) and the information about micelle structure was used to understand the interaction between the microemulsion and the selected waxes. The microemulsion was then tested during the restoration of the frescoes in the Major Chapel of the Santa Croce Basilica in Florence, Italy. After some preliminary tests on fresco mockups reproduced in the laboratory, the nanostructured fluid was successfully used to clean some wax deposits from the real paintings, hardly removable with traditional physico-mechanical methods.

Restoration of paper artworks with microemulsions confined in hydrogels for safe and efficient removal of adhesive tapes

The presence of pressure-sensitive tapes (PSTs) on paper artworks, either fortuitous or specifically applied for conservation purposes, is one of the most frequent and difficult issues encountered during restoration. Aged PSTs can damage or disfigure artworks, compromising structural integrity, readability, and enjoyment. Current procedures are often inherently hazardous for artistic media and paper support. Challenged by the necessity to remove PSTs from a contemporary and an ancient drawing (20th century, by artists da Silva and Hayter, and a 16th-century drawing of one figure from the Sistine Chapel by Michelangelo), we addressed this issue from a physicochemical perspective, leveraging colloid and interface science. After a characterization of the specific PSTs present on the artifact, we selected a highly water-retentive hydrogel as the host of 23% wt/wt of "green" organic solvents uniformly dispersed within the gel in the form of nanosized droplets. The double confinement of the organic solvent in the nanodroplets and into the gel network promotes a tailored, controlled removal of PSTs of different natures, with virtually no interaction with the solvent-sensitive artwork. This noninvasive procedure allows complete retrieval of artwork readability. For instance, in the ancient drawing, the PST totally concealed the inscription, "di mano di Michelangelo" ("from Michelangelo's hand"), a possibly false attribution hidden by a collector, which is now perfectly visible and whose origin is currently under investigation. Remarkably, the same methodology was successful for the removal of aged PST adhesive penetrated inside paper fibers of a drawing from the celebrated artist Lucio Fontana.